This invention relates to rotatable turbine blades and more particularly to a group of turbine blades having common platform and shroud portions with multiple, side entry roots extending therefrom.
The control stage in many axial flow steam turbines utilizes pinned root, multiple blade units because of their higher rigidity and lower vibration susceptibility than single blade units which each have a separate root. The pinned root, multiple blade units have given better performance when exposed to vibratory excitation, but have resulted in high control stage assembly costs.
Some pinned root turbine blades have a radially outer point of fixity during blade vibration, which is relatively distant from the blade's platform portion. With such a relatively large distance between the point of fixity and the platform portion, the effective, unsupported blade length is increased causing low blade frequencies to be obtained which can result in vibratory resonance during partial admission operation. In addition, low blade frequencies cause vibration of the blades to decay slowly and thus have a greater tendency to resonate.
A number of multi-rooted and multi-bladed control stage groups have been known and used in the past. Typical of these is a design having two roots on each blade section with several of the blade sections being "tied together" into one blade group by securing a shroud on the radially outer end of the blades. Securing the shroud to the blades was often accomplished by riveting or deforming tenons protruding from the blades through the shroud.
Disadvantages of such a design include low frequency blades due to the close circumferential proximity of the roots on each blade section and the possible disproportionate loading of such roots if a blade becomes cocked due to the circumferential thermal expansion of the shroud. A further example of the designs previously alluded to include the subject of ASME paper number 61-WA122 which suggests brazing a number of blade sections together for the purpose of obtaining higher rigidity. This design, however, due to the relatively long blade group formed after brazing and the relatively widely spaced roots results in a relatively low excitation frequency during partial admission operation. In addition, brazing separate blade sections together presents the possibility of introducing flaws into the joints and having those flaws propagate onto cracks resulting in eventual separation during turbine operation. Such separation could cause long and expensive turbinegenerator forced outage.
A desirable control stage blade structure would have a low cost of assembly in the high pressure turbine section, high blade rigidity, and a sufficiently high blade frequency to prevent resonance during partial admission operation of the turbine.